I installed 6.0 today but ran into a server startup error. The only thing I did was to paste the authentication token. Here's some output:

java.lang.RuntimeException: java.io.FileNotFoundException: C:\Users\Daniel\.BuildServer\config\main-config.xml (The requested operation cannot be performed on a file with a user-mapped section open) at jetbrains.buildServer.util.FileUtil.processXmlFile(FileUtil.java:169) at jetbrains.buildServer.serverSide.versioning.VersionManager.writeVersionIntoMainConfigFile(VersionManager.java:192) at jetbrains.buildServer.serverSide.versioning.VersionManager.writeVersionToDisk(VersionManager.java:167) at jetbrains.buildServer.serverSide.versioning.ConvertPerformer.writeDiskVersion(ConvertPerformer.java:104) at jetbrains.buildServer.serverSide.versioning.ConvertPerformer.runConverter(ConvertPerformer.java:96) at jetbrains.buildServer.serverSide.versioning.ConvertPerformer.runConverters(ConvertPerformer.java:78) at jetbrains.buildServer.serverSide.versioning.VersionManager.upgradeIt(VersionManager.java:391) at jetbrains.buildServer.serverSide.versioning.VersionManager.checkVersionsAndUpgradeSchemaIfNeeded(VersionManager.java:359) at jetbrains.buildServer.serverSide.versioning.VersionManager$2.run(VersionManager.java:415) at jetbrains.buildServer.serverSide.db.TeamCityDatabaseManager.withDB(TeamCityDatabaseManager.java:568) at jetbrains.buildServer.serverSide.versioning.VersionManager.ensureActualDatabase(VersionManager.java:411) at jetbrains.buildServer.rootDispatcher.TeamCityDispatcherServlet.processStartupServerInternal(TeamCityDispatcherServlet.java:437) at jetbrains.buildServer.rootDispatcher.TeamCityDispatcherServlet.processStartupServerSafe(TeamCityDispatcherServlet.java:23) at jetbrains.buildServer.rootDispatcher.TeamCityDispatcherServlet.access$400(TeamCityDispatcherServlet.java:191) at jetbrains.buildServer.rootDispatcher.TeamCityDispatcherServlet$5.run(TeamCityDispatcherServlet.java:2) at java.lang.Thread.run(Unknown Source)caused by: java.io.FileNotFoundException: C:\Users\Daniel\.BuildServer\config\main-config.xml (The requested operation cannot be performed on a file with a user-mapped section open) at java.io.FileOutputStream.open(Native Method) at java.io.FileOutputStream.<init>(Unknown Source) at java.io.FileOutputStream.<init>(Unknown Source) at jetbrains.buildServer.util.FileUtil.saveDocument(FileUtil.java:175) at jetbrains.buildServer.util.FileUtil.processXmlFile(FileUtil.java:166) at jetbrains.buildServer.serverSide.versioning.VersionManager.writeVersionIntoMainConfigFile(VersionManager.java:192) at jetbrains.buildServer.serverSide.versioning.VersionManager.writeVersionToDisk(VersionManager.java:167) at jetbrains.buildServer.serverSide.versioning.ConvertPerformer.writeDiskVersion(ConvertPerformer.java:104) at jetbrains.buildServer.serverSide.versioning.ConvertPerformer.runConverter(ConvertPerformer.java:96) at jetbrains.buildServer.serverSide.versioning.ConvertPerformer.runConverters(ConvertPerformer.java:78) at jetbrains.buildServer.serverSide.versioning.VersionManager.upgradeIt(VersionManager.java:391) at jetbrains.buildServer.serverSide.versioning.VersionManager.checkVersionsAndUpgradeSchemaIfNeeded(VersionManager.java:359) at jetbrains.buildServer.serverSide.versioning.VersionManager$2.run(VersionManager.java:415) at jetbrains.buildServer.serverSide.db.TeamCityDatabaseManager.withDB(TeamCityDatabaseManager.java:568) at jetbrains.buildServer.serverSide.versioning.VersionManager.ensureActualDatabase(VersionManager.java:411) at jetbrains.buildServer.rootDispatcher.TeamCityDispatcherServlet.processStartupServerInternal(TeamCityDispatcherServlet.java:437) at jetbrains.buildServer.rootDispatcher.TeamCityDispatcherServlet.processStartupServerSafe(TeamCityDispatcherServlet.java:23) at jetbrains.buildServer.rootDispatcher.TeamCityDispatcherServlet.access$400(TeamCityDispatcherServlet.java:191) at jetbrains.buildServer.rootDispatcher.TeamCityDispatcherServlet$5.run(TeamCityDispatcherServlet.java:2) at java.lang.Thread.run(Unknown Source)

Ok it seems I was able to solve this problem myself. Am I a great hacker or what? :^O Anyway here's what I did:

Changed version number in version.dat from 383 to 384. The log file was complaining that "Data directory format version: 383" and database version was 384.

This alone didn't help. It seemed the number 383 was also somewhere else: "TeamCity data format version mismatch: configuration version is 383 but database version is 384." So I found the number 383 in main-config.xml, changed it to 384 then restarted server again.

Entered new server authentication token from teamcity-server.log.

Success!

Lessons learnt:

Always backup data directory prior to server upgrade. For me this is C:\Users\Daniel\.BuildServer. Right-click drag&drop is so easy.